The
signing part is not in any build of Vista, and, as of today, to
restore the AppInit_DLLs functionality there is one more registry
key, named LoadAppInit_DLLs with the value of 1 that
needs to be created for the other key to take effect.

For
32-bit apps on a 64-bit OS you need to set this value under HKLM\Software\Wow6432Node\...